Surveying - Most Important topics for Civil Engineering Students

Surveying

Civil Engineering

Geodetic survey:

•  In this survey, the curvature of earth is taken into account in order to have a higher degree of precision. Such surveys are required for surveying large areas and measurements.
• As to survey a country (large area) geodetic surveying is required.

Plain Survey

• Reconnaissance Surveying: The reconnaissance survey is an extensive study of an entire area that might be used for a road or airfield. Its purpose is to eliminate those routes or sites which are impractical or unfeasible and to identify the more promising routes or sites. Existing maps and aerial photographs may be of great help.
• Curve joining two intersecting lines which lie in a horizontal plane is called horizontal curve
• Curve joining two intersecting lines which lie in a vertical plane is called vertical curve.

Well-conditioned triangle:

• A triangle is stated to be a well-conditioned triangle when all angle in it is more than 30° or lesser than 120°.
• A triangle in which an angle of the triangle is less than 30° or greater than 120° is known as ILL conditioned triangle.
• Equilateral Triangle is the best-suited triangle in Chain Triangulation or Chain Surveying.
• Well-conditioned triangles are preferred because their apex points are very sharp and can be located by a single ‘dot’. In such a case, there is no possibility of relative displacement of the plotted point.

GIS 

• GIS stands for the geographic information system. These are programs or a combination of programs that work together to help users make sense of their spatial data. This includes management, manipulation and customization, analysis, and creating visual displays.

• Spatial data, also known as geospatial data, is a term used to describe any data related to or containing information about a specific location on the Earth’s surface. It exists in a variety of formats and contains more than just location-specific information.

G.I.S has following components

• 1. Hard ware (computer)

• 2. Software

• 3. Data management

• 4. End User and

• 5. Methods

Level Surface

• Level surface is the continuous surface parallel to the mean spheroid of the earth. The line representing the level surface is termed as level line.

• The level line makes right angles to the vertical line or plumb line at any point. It means the any point on level line is equidistance from the center of earth. Even though it is a curved surface, it is considered as plane surface for smaller area works.

• Whereas the line of sight is the straight line perpendicular to the plumb line at the telescope.

Temporary Adjustment of SURVEYING INSTRUMENTS

• The adjustments to be made at every setting of the instrument are called temporary adjustments.

• Following are the five temporary adjustments are required:

• 1. Setting up: Tripod stand is set on the ground firmly so that its top is at a convenient height.

• 2. Centering: It is done to place the vertical axis exactly over the station mark

• 3. Levelling: The levelling is done to make the vertical axis of the instrument truly vertical or to make horizontal plate truly horizontal

• 4. Focusing the eye piece: The eye piece is focused to make the cross hairs distinct and clear

• 5. Focusing the objective: This is done to bring the image of the object in plane of the cross hairs

Total Station:

• Total station is a combination of an electronic theodolite and an electronic distance meter (EDM).

• The combination makes it possible to determine the coordinates of a reflector by aligning the instruments cross-hairs on the reflector.

• Simultaneously it can measure vertical and horizontal angle and slope distance.

• A microprocessor in the instrument take care of the recordings, readings, and other computations.

LATITUDE AND DEPARTURE

• Latitude: Projection of a line on N - S direction called latitude.

• Departure: Projection of a line on E – W direction is called Departure.

• Where, Σ L = sum of all latitudes, Σ D is sum of all departures.

• ΣL=l1×cosθ1+l2×cosθ2+l3×cosθ3+l4×cosθ4

• ΣD=l1×sinθ1+l2×sinθ2+l3×sinθ3+l4×sinθ4

ERRORS AND THEIR REMOVAL IN SURVEYING

• The process of adjusting the latitudes and departures to make the algebraic sum of latitudes or departures to zero is called the balancing of errors.

• The two types of balancing rules to eliminate errors in traverse surveying are as follows:

• 1) Bowditch rule (Compass rule): It is most commonly adopted when angular measurement and linear measurement both are nearly of same precision. The correction is considered directly proportional to the length of the side.

• By Bowditch rule Correction to a particular line is given by

• Correction for Latitude/Departure =

• Total error of Latitude or Departure × length of the side/perimeter of the traverse

• 2) Transit rule: When angular measurements are more precise than linear measurement, the transit method is adopted.

Error due to incorrect chain:

• If the length of the chain used in measuring the length of the line is not equal to the true length or the designated length, the measured length of the line will nor be correct and suitable correction will have to be applied.

• If the chain is too long, the measured distance will be less. The error will, therefore, be negative and the correction is positive.

• Similarly, if the chain is too short, the measured distance will be more, the error will be positive and the correction will be negative.

Let,

L = True or designated length of the chain or tape.

L' = Incorrect (or actual) length of the chain or tape used.

I' = Measured length of the line

I= Actual or true length of the line.

Correction to a measured length :

The true length of line  =  measured  length of  a line × L′/L

TYPES OF CHAIN

S.No.	Type of chain	Length	No. of links	Length of links
1	Meter Chain	20 m or 30 m	100 or 150	20 cm
2	Engineering Chain	100 ft	100	1 ft
3	Gunter Chain	66 ft	100	0.66 ft
4	Revenue Chain	33 ft	16	2116$\mathrm{2 }\frac{\mathrm{1/}}{16}$ ft

(As 1 foot = 0.3048 m)

66 feet = 0.3048 × 66 m = 20.11 m

 Gunter’s chain is 20.12 m long.

IMP TERMS RELATED TO SURVEY

• Latitude: It is the length (l) of projection of a line on to the North-South (l cosθ) axis.

• Northing  Positive, Southing  Negative

• Departure: It is the projection of the line (l) on to the east-west (l sinθ) axis.

• Easting  Positive, Westing Negative

• Important Points:

• Closing Error √(∑L)2+(∑D)2

• Relative Precision e/P

• Angle of Error tanθ=∑D/∑L

Chain Survey

• Chain surveying is the simplest method of surveying.
• Measurements for this survey are taken on the field and other supplementary works like plotting and calculations are carried out in office.
• The measurements in chain surveying are linear.
• Chain, tape, ranging rod, arrows and, cross staff are the tools required for chain surveying.
• It is used when plans are required on a large scale.

It can be used if the area meets the following conditions:

• 1. The area shall be fairly small.
• 2. The ground is fairly leveled and open with simple details
• 3. The area needs to be open.

Transition curve:

• To minimize discomfort arising out of the sudden change in curvature at the junction of a tangent and a curve, a special type of curve is provided in between for gradual change from the back tangent to the circular curve and again from the circular curve to the forward tangent. This horizontal curve having varying radius is known as the transition curve.

There are five objectives for providing transition curve and are given below:

• 1. To introduce gradually the centrifugal force between the tangent point and the beginning of the circular curve.

• 2. Gradual increase of super-elevation from zero at the junction of the transition curve with the main curve to the specified amount at the point of tangency.

• 3. To provide a gradual introduction of extra widening.

• 4. A gradual change of gradient from zero at the junction of the transition curve with the main curve to the specified amount at the point of tangency.

• 5. A gradual change of radius from some value at the junction of the transition curve with the main curve to the infinite value at the point of tangency.

• It means that it is a gradual decrease of curvature from zero at the tangent point to the specified quantity at the junction of the transition curve with the main curve

SETTING OUT A CURVE

• Locating various points along the length of the curve at equal and convenient distances is known as “setting out a curve”.

Methods employed for setting out a circular curve are:

• 1. Rankine Method of tangential angles: Rankine method is based on the principle that the deflection angle to any point on a circular curve is measured by one half the angle subtended by the arc from Point of curve to that point.

• 2. Two theodolite method: This method is used when the ground is unsuitable for chaining and is based on the principle that the angle between the tangent and the chord is equal to the angle which that chord subtends in the opposite segment.

• 3. Tacheometric method: this method is less accurate than “Rankine Method of tangential angles”.

Theodolite

• Lower Plate is a horizontal circular plate that provides the main scale reading of a horizontal angle and a means to fix or unfix the whole instrument.

• Upper Plate is a horizontal circular plate having two diametrically opposite vernier scales and it provides a means to fix or unfix the upper plate of the instrument with its lower plate.

• To change the reading on the circle while measuring an angle following procedure is adopted

• Tighten the lower clamp ⇒ Loosen the upper clamp ⇒ Turn the instrument and direct the telescope towards the target to bisect it accurately with the use of tangent screw ⇒ Read the Vernier and record the readings

• ∴ To change the reading on the circle while measuring an angle, upper clamp is loosened and lower clamp is tightened is correct statement.

Laser theodilites:

• The laser theodolite is either a purpose-built instrument or a standard theodolite converted into a laser theodolite using a laser eyepiece attachment.

• The laser beam projected by these theodolites coincides exactly with the line of collimation and is focused using the telescope focusing screw to appear as a dot in the centre of the crosshairs.

• This instrument can be used in place of conventional theodolite in almost any alignment or intersection technique.

• They have all the functions of electronic theodolite with facility of more accurately bisecting the target.

• Hence it is useful in major construction where alignment is to be done precisely and quickly.

Important points:

• 1. The size of a theodolite is defined by the diameter of the graduated circle of the lower plate. .
• 2. The process of establishing intermediate points, on a given straight line whose ends are intervisible is done with a theodolite is called lining in.
• 3. Removal of parallax may be achieved by refocusing the eyepiece and the objective.
• 4. Line of collimation: It is an imaginary line passing through the intersection of the crosshairs at the diaphragm and the optical center of the object-glass and its continuation.
• 5. Axis of Bubble tube: It is an imaginary line tangential to the longitudinal curve of the bubble tube at its midpoint.
• 6. Axis of a telescope: This axis is an imaginary passing through the optical center of the object-glass and the optical center of the eye-piece.

Simple Curve

• While setting out simple curves, pegs are to be fixed along the required curve at suitable intervals.

• As it is difficult to measure along the curve, the chord length are taken as curved length for fixing the curve.

• Chord length for peg interval is kept between 1/10th and 1/20th of the radius of curve to achieve the relative precision.

• When it is 1/10th of R, the error is 1 in 2500 and if it is 1/20th R, the error is 1 in 10,000.

• In practice the radius of the curve varies from 200 m to 1000 m and hence the chord length of 20 m is reasonably sufficient.

• For greater accuracy it may be taken as 10 m, the most appropriate answer is R/20.

While setting out simple curves, pegs are to be fixed along the required curve at suitable intervals.

• As it is difficult to measure along the curve, the chord length are taken as curved length for fixing the curve.
• Chord length for peg interval is kept between 1/10th and 1/20th of the radius of curve to achieve the relative precision.
• When it is 1/10th of R, the error is 1 in 2500 and if it is 1/20th R, the error is 1 in 10,000.
• In practice the radius of the curve varies from 200 m to 1000 m and hence the chord length of 20 m is reasonably sufficient.
• For greater accuracy it may be taken as 10 m, the most appropriate answer is R/20.

COMPASS USED IN SURVEYING

Parameter	Surveyor’s  Compass	Prismatic Compass
Magnetic Needle	Edge bar needle is  used and it act as index	Broad needle is used  and it is hidden below the aluminium ring  and does not act as index.
Graduation	Graduated ring is attached  to the box and ring rotates with box. Graduations vary from 00 to 900  in the four quadrants.  Zero points are marked with N and S.	Graduated ring attached to the needle and remains stationary. Graduations vary from  00 to 3600 clockwise  with zero at south end.
SightingVanes	Readings are taken by directly looking  through the glass  and reading the north end of needle.	Readings are taken  through the vertical  side of the prism provided  at the eye vane.
Reading	Sighting and reading are done separately  and that too from different positions. After sighting the object,  the observer move around and takes the reading at North end of needle.	Sighting and reading  are done simultaneously.
Support-Requirement	Tripod is compulsory.	Tripod may or may not be used.

Laser theodilites:

• The laser theodolite is either a purpose-built instrument or a standard theodolite converted into a laser theodolite using a laser eyepiece attachment.

• The laser beam projected by these theodolites coincides exactly with the line of collimation and is focused using the telescope focusing screw to appear as a dot in the centre of the crosshairs.

• This instrument can be used in place of conventional theodolite in almost any alignment or intersection technique.

• They have all the functions of electronic theodolite with facility of more accurately bisecting the target.

• Hence it is useful in major construction where alignment is to be done precisely and quickly.

Important points:

• 1. The size of a theodolite is defined by the diameter of the graduated circle of the lower plate. .
• 
  
• 2. The process of establishing intermediate points, on a given straight line whose ends are intervisible is done with a theodolite is called lining in.
• 
  
• 3. Removal of parallax may be achieved by refocusing the eyepiece and the objective.
• 
  
• 4. Line of collimation: It is an imaginary line passing through the intersection of the crosshairs at the diaphragm and the optical center of the object-glass and its continuation.
• 
  
• 5. Axis of Bubble tube: It is an imaginary line tangential to the longitudinal curve of the bubble tube at its midpoint.
• 
  
• 6. Axis of a telescope: This axis is an imaginary passing through the optical center of the object-glass and the optical center of the eye-piece.Laser theodilites:

Remote sensing:

• Remote sensing is the process of detecting and monitoring the physical characteristics of an area by measuring it's reflected and emitted radiation at a distance typically from satellite or aircraft. Special cameras collect remotely sensed images, which help researchers "sense" things about the Earth.

Types of remote sensing

1.Active Remote Sensing

• Active sensors emit energy to scan objects and areas and a sensor then detects and measures the radiation that is reflected or backscattered from the target.

• Examples:  RADAR and LiDAR 

2.Passive Remote Sensing

• Passive sensors gather radiation that is emitted or reflected by the object or surrounding areas. Reflected sunlight is the most common source of radiation measured by passive sensors.

• Examples: film photography, infrared, charge-coupled devices, and radiometers.

TYPES OF SURVEYING

 

PLANE TABLE SURVEYING

• Plane table surveying is the graphical method of survey in which the field observations and plotting are done simultaneously.

• Plane table surveying is based on the principle that lines drawn during plotting always lie parallel to the corresponding lines actually present on the ground i.e the principle of plane table surveying is parallelism.

Advantages:

• The observations and plotting are done simultaneously
• The errors and mistakes in plotting can be checked by drawing check lines
• Irregular objects can be plotted accurately
• No great skills are required
• It is less costly than a theodolite surveying
• It is advantageous in magnetic areas

Disadvantages:

Not suitable for work in a wet climate and in a densely wooded country

Difficult to replot the map to a different scale

It does not give highly accurate results

• Traversing and Resection are the methods used for locating the position of instrument station on the drawing.

• Radiation and intersection are the methods used for plotting the position of objects on the drawings.

Other points to be kept in mind:

• Resection is a method of plane table surveying in which location of the plane table is unknown and it is determined by sighting it to known points or plotted points. It is also called the method of orientation.

Four methods of Plane table surveying are:

1. Resection

• It is a method of orientation employed when the table occupies a position not yet located on the drawing sheet. It is defined as the process of locating the instrument station occupied by the plane table by drawing rays from the stations whose positions have already been plotted on the drawing sheet.

Methods generally employed in Resection are:

It can be conducted by:

• Resection by compass
• Resection by back sighting
• Resection by two points (two points problem)
• Resection by three points (three points problem)

2. Intersection

3. Traversing

4. Radiation

Optical Square: 

• It is more convenient and accurate than cross-staff for setting out right angles. Consists of two mirrors making a 45° with each other, one mirror totally silvered another top-bottom un-silvered.

• To set a right angle: To set a right angle on a survey line, the instrument is held on the line with its centre on the point at which perpendicular is erected. The silts F and G are directed towards the ranging rod fixed at the end of the line.

• The surveyor then directs person, holding a ranging rod and stationing in a direction roughly perpendicular to the chain , to move till the two images described above coincides.

• ∴ For taking offsets with an optical square on the right hand side of the chain line it is held by left hand upright.

Accuracy

• The accuracy of a set of repeated observations is being defined as amount of closeness of their mean to the population or distribution mean, i.e., closeness of the mean of observations to the true value.

Degree of accuracy

• The degree of accuracy indicates the accuracy attained in the measurements.
• t is usually expressed as the ratio of the error and the associated measured value.
• For example, a degree of accuracy of 1 in 10,000 indicates that there is an error of 1 unit in 10,000 units of measured / observed value.

Note:

For linear measurement:

• The degree of accuracy of linear measurement is usually expressed as the ratio of the probable error and the measured distance.
• For example, if there is a probable error of ± 0.05 m in a measured distance of 584.65 m, the degree of accuracy is 1 in 11700 as explained below.

For angular measurement:

• For angular measurements, the degree of accuracy is usually expressed as k × √N, where N = Number of angles measured

• The angular measurement taken in compass surveying has the error should not be more than 5 minutes. Thus, the limit of accuracy is not more than 5 minutes.

Limits of Errors in Chaining:

• 1. For measuring over rough or hilly ground, the permissible error in chaining is 1 in 250.

• 2. In an Ordinary Chain Survey, the maximum permissible error is 1 in 1000.

• 3. If a steel tape or a steel band chain is used to obtain greater accuracy in measurements, the limiting error may be 1 in 2000.

• 4. When the standardized steel or invar tape is used and corrections for pull, temperature, sag, slope and alignment etc. are applied, the maximum error should not exceed 1 in 5000.

1. Compensating errors 

• These are those which remain after mistakes and systematic errors have been eliminated and are caused by the combination of errors beyond the ability of the observer to control.
• They are proportional to the square root of the length of the line.

2. Accidental errors

• They represent the limit of precision in the determination of a value.
• They obey the law of chance and must be handled according to the mathematical law of probability.
• These errors are proportional to the square root of the length of the line (√L).

3. Cumulative Errors/Systematic errors  

• The errors that occur in the same direction and which finally tend to accumulate are said to be Cumulative errors.
• They are cumulative in nature. Examples of systematic errors are Collimation in a level, Expansion of steel tape, etc.
• They are proportional to the length of the line.

4. Random errors 

• These are all those discrepancies remaining after the mistakes and systematic errors are removed.
• It is mainly caused by the limitations of observer and instruments and is random in nature.

Geodetic Survey:

• The surveys in which the curvature of the earth is taken into consideration are known as geodetic surveying.

• All lines lying on the surface are curved lines and the triangles are spherical triangles. It is considered to be geodetic when the limit of the survey is more than 250 km2.

• ∴ 250 square kilometre is the upper limit of the survey area for the use of a plane survey.

• The theodolite measures horizontal angles in the horizontal plane, but when the area becomes large, such as in the case of primary triangulation, the curvature of the earth means that such planes in large triangles called as spherical triangles or geodetic triangles are not parallel at the apices.

• The three angles of a large triangle do not total 180°, as in the case of plane triangles, but to 180° + ε, where ε is known as spherical excess.

• ∴ The sum of angles (degree) of a spherical triangle should be greater than 180° and should be less than 540°.

ADDITION DETAILS

• Distance between two points is measured by counting the number of normal steps which will cover the distance between two points along a straight line.
• Width of each step depends as follow:
• Steps will be shorter in tall vegetation than in short vegetation.
• Steps will be shorter walking uphill than walking downhill.
• Steps will be shorter walking on sloping ground than on flat ground.
• Steps will be shorter walking on soft ground than on hard ground.
• The distance between steps for measuring downhill to obtain better accuracy _____.
• decreases with increase of slope

Compass Surveying:

Compass surveying is the branch of surveying in which the position of an object is located using angular measurements determined by a compass and linear measurements using a chain or tape. Compass survey is done for large areas. Hence, locating the points and traversing the area are the main objectives of compass surveying.

The principle of Compass surveying is traversing, which involves a series of connected lines.

• Chain surveying is used when the area to be surveyed is relatively flat and small.
• Traversing is used when the area is large and undulations are present.
• In compass surveying, two types of the compass are used viz. Prismatic and Surveyor’s Compass.

Correction due to refraction = 1/7(Correction due to curvature)

CONTOUR LINES

• Contour lines are the curves joining equal elevation points.
• The contour lines are always closed curves, it may not be visible complete on a map.
• They never intersect each other (except cliffs, they might merge or intersect).
• Evenly spaced contour lines indicate a uniform slope.
• Closely spaced contour lines show a steep slope.
• Widely spaced contour lines show gentle slope.
• Uneven spacing – irregular or variable slope.
• Closed contours with arrow in the centre represent depressions.

Important Points:

• Horizontal distance between contour lines is inversely proportional to slope.
• Uniform slopes have uniformly spaced lines.
• Along plane surfaces, contour lines are straight and parallel.
• Contour lines are perpendicular to lines of steepest slopes.
• For summits or depressions, contour lines most close upon themselves.
• A single contour line cannot lie between two lines of higher or lower elevation.

Representative Fraction (R.F.):

• It is a ratio or fraction, that expresses the mathematical relationship between map and land. It is the ratio of the distance between any two fixed points on map to the original distance on the land.

• For example , 1 cm = 50 m is represented as 1:5000 or 1/5000

• Scale (S): Scale for the above representative fraction would be given as 1 cm to a meter. (as 1 m = 100 cm

• Let suppose the denominator increases from 100 to 1000

• Scale corresponding to R.F of 1: 10000 would be given as 0.1 cm to a meter.

• ∴ With an increase in the denominator of the representative fraction, the scale of the map will decreases.

We know that for levelling work, following equation must be satisfied:

∑BS−∑FS=LastRL−FirstRL

HORIZONTAL DISTANCE MEASURED BY TECHEOMETRY

In Tacheometry the multiplying constant ‘k’ is the ratio of focal length of objective lens and stadia hair Additive constant ‘c’ is the sum of focal length of object and and distance of bject lens to center of the instrument. The distance (H) is calculated as

H = k × S + C

TERMS RELATED TO THEODOLITE

• A telescope is a focusing instrument that has an object piece at one end and eyepiece at the other end. It is fitted on the theodolite which is used to measure horizontal and vertical angles.

• Chromatic aberration: The defect of a lens whereby rays of white light proceeding from a point get dispersed into their components and conveyed to various foci, forming a blurred and colored image is known as Chromatic aberration.

• Achromatism: The achromatic telescope is a refracting telescope that uses an achromatic lens to correct for chromatic aberration.

• Spherical aberration: It is an aberration that may occur in both the case of lens and mirrors. Here, light rays nearer the optical axis are refracted, or reflected, differently, from light rays further away from it. In telescopes, this aberration is seen as a blurring of the image.

• दूरबीन एक फोकसिंग यंत्र है जिसके एक सिरे पर ऑब्जेक्ट पीस होता है और दूसरे छोर पर नेत्रक होता है। इसे थियोडोलाइट पर फिट किया जाता है जिसका उपयोग क्षैतिज और ऊर्ध्वाधर कोणों के मापन के लिए किया जाता है।
• 
  
• वर्ण विपथन​: एक लेंस का दोष जिससे एक बिंदु से निकलने वाली सफेद प्रकाश की किरणें उनके घटकों में फैल जाती हैं और विभिन्न फोकी तक जाती हैं, जिससे एक धुंधली और रंगीन छवि बनती हैं जिसे वर्ण विपथन के रूप में जाना जाता है।
• 
  
• अवर्णता​: वर्णहीन दूरबीन एक अपवर्तक दूरबीन है, जो वर्ण विपथन में सुधार करने के लिए वर्णहीन लेंस का उपयोग करती है।
• 
  
• गोलीय विपथन: यह एक विपथन है जो लेंस और दर्पण दोनों के ही मामले में हो सकता है। यहां, प्रकाश किरणें प्रकाशीय अक्ष के निकट प्रकाश किरणों से दूर अलग प्रकार से अपवर्तित या परावर्तित होती हैं। दूरबीनों में, इस विपथन को छवि के धुंधलापन के रूप में देखा जाता है।

Vernier Scale is used to measure the fractional part of the least division marked on the main scale.

Types of Vernier Scale

Direct Vernier:

• N divisions of the direct vernier = (N - 1) division of the main scale

Retrograde Vernier:

• N divisions of the retrograde vernier = (N + 1) division of the main scale

Extended Vernier:

• N divisions of the extended vernier = (2 N - 1) division of the main scale

THEODOLITE

• 1. The size of the theodolite is defined by the diameter of the graduated circle of the lower plate.

• 2. The process of establishing intermediate points, on a given straight line whose ends are intervisible is done with a theodolite is called lining in.

• 3. Removal of parallax may be achieved by refocusing the eyepiece and the objective.

• 4. Line of collimation: It is an imaginary line passing through the intersection of the crosshairs at the diaphragm and the optical center of the object-glass and its continuation.

• 5. Axis of Bubble tube: It is an imaginary line tangential to the longitudinal curve of the bubble tube at its midpoint.

• 6. Axis of a telescope: This axis is an imaginary passing through the optical center of the object-glass and the optical center of the eye-piece.

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